FIG. 25 is a diagram for illustrating a method for estimating a welded region 2 of a welded object 1 welded by a conventional resistance spot welding. As shown in FIG. 25, the welded object 1 is formed by spot-welding an upper plate 3 and a lower plate 4 together at the welded region 2. In the welded region 2, the upper plate 3 and the lower plate 4 are welded together due to disappearance of an interfacial portion 5, caused by melting, between the upper plate 3 and the lower plate 4. In non-welded regions 6 except for the welded region 2 of the welded object 1, the interfaces 5 remain between the upper plate 3 and the lower plate 4.
The welded region 2 includes a nugget portion 7, in which the upper plate 3 and the lower plate 4 are melted and welded together, and a corona-bonded portion 8, which covers the nugget portion 7 and in which the upper plate 3 and the lower plate 4 are slightly melted and closely adhered together. In the resistance spot welding, a front face 9 and a rear face 10 of the welded object 1 are arranged to be substantially parallel with each other.
As a conventional art, there is a method for estimating the welded region 2 of the welded object 1 by using an ultrasonic wave. In the conventional art, an ultrasonic probe 11 adapted to generate the ultrasonic wave is scanned to pass through over the welded region 2 so as to take therein a reflected wave of the ultrasonic wave reflected from the welded object 1 for each scanning displacement. The reflected wave 12 of the ultrasonic wave introduced in and reflected from the welded object 1 will be a reflected wave reflected from a bottom face 13 of the upper plate 3 in the non-welded regions 6, while it will be a reflected wave reflected from a bottom face 14 of the lower plate 4 in the welded region 2. In this conventional estimating method, boundary positions 15 between the welded region 2 and the non-welded regions 6 are estimated by comparing the reflected waves from the upper plate 13 and from the lower plate 14, thus estimating a size of the welded region 2.
In a technique disclosed in Patent Document 1, as an estimation of the welded region 2 in the welded object 1, the nugget portion 7 is obtained based on attenuation of multiple reflection waves multiply reflected from a top face 16 of the upper plate 3 and from the bottom face 14 of the lower plate 4. In a technique disclosed in Patent Document 2, as the estimation of the welded region 2 in the welded object 1, the nugget portion 7 is obtained based on a level of a transverse ultrasonic wave generated by mode conversion that is caused when an ultrasonic wave is reflected by the bottom face 14 of the lower plate 4.
As another conventional art, there is a method for estimating a joined region by employing an ultrasonic wave, the joined region being formed by a continuous friction stir joining. In a technique disclosed in Patent Document 3, presence of holes indicative of a defect of joining in the joined region is detected when amplitude of a bottom face echo reflected from a bottom face of a joined object is lower than a theoretical value. It is noted that the joined object formed by the continuous friction stir joining has a substantially flat front face.    Patent Document 1: JP 3-233352 A    Patent Document 2: JP 2000-146928 A    Patent Document 3: JP 2004-317475 A
As one of joining methods, there is a lap-joint-joining method utilizing the friction stir joining method. Namely, in a lap joint formed by the friction stir joining method, the upper plate and the lower plate are joined together due to disappearance of the interfacial portion, which was stirred between the upper plate and the lower plate. Conventionally, the joined region and the joining strength of a lap-joint-joined object formed by the friction stir joining method are obtained by a destructive inspection, respectively. Therefore, there still remains a need for a method and an apparatus for obtaining the joined region and the joining strength by a non-destructive inspection also in the case of the lap-joint joined object formed by the friction stir joining method.
However, the joined object formed by the friction stir joining method generally includes a non-flat tool-processing or tool-plunging face having a complex concave/convex shape. Therefore, in the technique of the non-destructive inspection employing the reflected wave reflected from the bottom face 14 of the bottom plate, the reflected wave is affected by such a concave/convex shape of the tool-plunging face. This makes it difficult to obtain the joined region and the joining strength with respect to an object formed by a spot friction stir joining method.